Scintillation counting involves the process of measuring radioactivity by means of converting radioactive energy into light pulses. A scintillator is a chemical used to convert radioactive pulses into light. When ionizing particles such as alpha, beta, or gamma rays are absorbed by the scintillator, some of the energy acquired by the scintillator is emitted as a pulse of visible or near ultra-violet light. The light is picked up by a photomultiplier tube and the combination of a scintillator and a photomultiplier tube is called a scintillation counter.
Typically, a gamma scintillation counter consists of a sodium iodide crystal with a well drilled into it in which a vessel containing the gamma emittor is placed. The gamma radiation passes through the walls of the vessel and into the crystal causing scintillation. For the counting of very low energy beta emittors such as .sup.3 H, .sup.14 C and .sup.35 S, the liquid scintillation counter was developed. According to procedures heretofore known in the art, the counting of these isotopes involves dissolving the radioactive isotope in a liquid solution containing the scintillator and placing the vessel containing this mixture into the instrument with a photomultiplier tube to observe the scintillations. The beta emittors emit an energy too weak to pass through the walls of any of the vessels so that the scintillator must be in very close proximity to the radioactive isotope.
Liquid scintillation counting has been used for several decades to count .sup.14 C and .sup.3 H. In the case of thin layer and paper chromatography, the strips have been customarily eluted with a liquid scintillation solution and subsequently counted. This method has entailed the accumulation of large volumes of toxic and radioactive solvents which require disposal at great expense. As will be seen, the present invention provides a method for directly counting paper or thin layer chromatographs and other treated carriers in a solid state, thereby eliminating the problems of disposal of large volumes of radioactive liquid waste.
Previous attempts have been made in the art to count radioactive substances without adding scintillation elutent. A method for counting .sup.32 P without added scintillation fluid by use of the Cerenkov radiation given off by the high energy emittor is well known in the literature. Likewise, the somewhat cumbersome method of detecting .sup.3 H or .sup.14 C by autoradiography is known and involves placing a photographic film adjacent to a sheet containing the radioactive material. Recent improvements in this technique have incorporated scintillators to mediate the photographic detection of radioactivity.
As will be seen from the following description, the present invention represents a significant improvement over the prior art methods for counting radioactive emissions. The method of solid phase scintillation counting which has been discovered provides numerous advantages which include the elimination of the possibility of contamination by radioactive liquid, the avoidance of accumulating large volumes of radioactive waste, and the allowance for direct re-use of containers without the need for cleaning.